TITLE:   GCN CIRCULAR
NUMBER:  29597
SUBJECT: GRB 210306A: Swift detection of a burst with a bright optical counterpart
DATE:    21/03/06 04:12:28 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

V. D'Elia (SSDC), E. Ambrosi (INAF-IASFPA), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), J. A. Kennea (PSU), N. J. Klingler (PSU),
S. Laha (GSFC/UMBC/CRESST), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 03:53:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210306A (trigger=1035994).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 129.965, +60.196 which is 
   RA(J2000)  =  08h 39m 52s
   Dec(J2000) = +60d 11' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks in a complex
structure with a duration of about 15 sec.  The peak count rate
was ~20,000 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 03:55:12.07 UT, 74.9 seconds after
the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source with a position: RA, Dec 129.96204, 
60.203271 which is equivalent to:
   RA(J2000)  = 08h 39m 50.89s
   Dec(J2000) = +60d 12' 11.7"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 27 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	08:39:49.95 = 129.95813
  DEC(J2000) = +60:12:19.4  =  60.20538
with a 90%-confidence error radius of about 0.61 arc sec. This position is 13.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.22 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.052. 

Burst Advocate for this burst is V. D'Elia (delia AT ssdc.asi.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)